The process of fermentation production of chemical products using sugars as raw materials has been used for producing various industrial materials. At present, as the sugars to be used as fermentation feedstocks, those derived from food materials such as sugar cane, starch and sugar beet are industrially used. However, in view of the fact that rise in the prices of food materials is expected due to future increase in the world population, or in an ethical view of the fact that sugars as industrial materials may compete with sugars for food, a process for efficiently producing a sugar liquid from a renewable nonfood resource, that is, cellulose-containing biomass, or a process for using an obtained sugar liquid as a fermentation feedstock to efficiently convert it to an industrial material, needs to be constructed in the future.
As the prior art for obtaining sugar from biomass, methods wherein concentrated sulfuric acid is used to hydrolyze cellulose and hemicellulose contained in the biomass into monosaccharides represented by glucose and xylose (Patent Documents 1 and 2), and methods wherein pretreatment is carried out for improving the reactivity of biomass, followed by hydrolysis of the biomass by enzymatic reaction (Patent Documents 3 and 4) are generally known.
However, in hydrolysis of a cellulose-containing biomass, decomposition of the cellulose and hemicellulose components and the like proceeds while decomposition reaction of produced sugars such as glucose and xylose proceeds to produce by-products such as furan compounds including furfural and hydroxymethylfurfural, and organic acids including formic acid and acetic acid, which is problematic. These compounds have inhibitory actions during the fermentation step using a microorganism and cause inhibition of the growth of the microorganism, leading to a decreased yield of the fermentation product. Therefore, these compounds are called fermentation inhibitors and have been seriously problematic when a cellulose-containing biomass sugar liquid was used as a fermentation feedstock.
As a method for removing such fermentation inhibitors during the sugar liquid production process, a method called overliming has been disclosed (Non-patent Document 1). In this method, during a step of neutralizing an acid-treated cellulose or saccharified liquid by addition of lime, the mixture is kept for a certain period with heating to about 60° C. to remove fermentation inhibitors such as furfural and HMF together with the gypsum component. However, overliming has only a small effect of removing organic acids such as formic acid, acetic acid and levulinic acid, which is problematic.
Further, as another method for removing fermentation inhibitors, a method wherein water vapor is blown into a sugar liquid prepared from a cellulose-containing biomass to remove fermentation inhibitors by evaporation has been disclosed (Patent Document 5). However, since such a method by evaporative removal is dependent on the boiling points of the fermentation inhibitors, the removal efficiencies for fermentation inhibitors such as organic acids having high boiling points are especially low, so that a large amount of energy is required to achieve sufficient removal efficiencies, which is problematic.
There is also a method wherein fermentation inhibitors are removed by ion exchange (Patent Document 6), but it is problematic in view of the cost. Further, there is a method wherein adsorptive removal is carried out using a wood-based carbide, that is, active carbon or the like, but the subjects to be removed are limited to hydrophobic compounds, which is problematic (Patent Document 7).
Further, there is a method in which fermentation inhibitors are removed with a membrane (Patent Document 8), but the amount of fermentation inhibitors that can be removed into the permeate side is limited, which is problematic.